Gram's stain

Gram's stain
Gram's stain or Gram stain 'gram- n
1) a method for the differential staining of bacteria that involves fixing the bacterial cells to a slide and staining usu. with the basic dye crystal violet, treating with an iodine solution to fix the dye, washing with acetone or alcohol to decolorize the dye, and counterstaining usu. with a red safranine dye which shows up only if the crystal violet has been decolorized, that results in gram-positive bacteria retaining the purple dye and gram-negative organisms having it decolorized so that the red counterstain shows up, and that is believed to depended on differences in the structure and composition of the bacterial cell walls which facilitate or prevent the decolorization of the crystal violet dye by the alcohol or acetone called also Gram's method
2) the chemicals used in Gram's stain

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a method of staining bacterial cells, used as a primary means of identification. A film of bacteria spread onto a glass slide is dried and heat-fixed, stained with a violet dye, treated with decolourizer (e.g. alcohol), and then counterstained with red dye. Gram-negative bacteria lose the initial stain but take up the counterstain, so that they appear red microscopically. Gram-positive bacteria retain the initial stain, appearing violet microscopically. These staining differences are based on variations in the structure of the cell wall in the two groups.
H. C. J. Gram (1853-1938), Danish physician

Medical dictionary. 2011.

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